1. GET BENT! (never gets old) 
2. Yukiko saying "You're wide open!" when scoring a crit (no one ever said anything about it, but every time i heard her say it i repeated it and laughed to myself) 
3. Aha! Is this our chance? (who wouldn't love this line?) 
4. I looooove them balls 
5. everyone still referring to Morooka as "King Moron" even after he was brutally killed 
6. You have welcomed Satan into your heart. 
7. I'm Jack Frost, ho! 
8. Anything and everything Funky Student says 
9. the "voices" of Ghoul, Black Frost, or any other persona or character 
10. when Kou and Daisuke have to clean up in the gym, having the option to "help out" or "be an ass and leave."

1. GET BENT! (never gets old) 
2. Yukiko saying "You're wide open!" when scoring a crit (no one ever said anything about it, but every time i heard her say it i repeated it and laughed to myself) 
3. Aha! Is this our chance? (who wouldn't love this line?) 
4. I looooove them balls 
5. everyone still referring to Morooka as "King Moron" even after he was brutally killed 
6. You have welcomed Satan into your heart. 
7. I'm Jack Frost, ho! 
8. Anything and everything Funky Student says 
9. the "voices" of Ghoul, Black Frost, or any other persona or character 
10. when Kou and Daisuke have to clean up in the gym, having the option to "help out" or "be an ass and leave."

Dude, "Get bent" in Vinny's faux-serious voice is fucking hilarious.  Every time.

Indeed. Get Bent never gets old.

That dude making those Youtube highlights needs to make more.

the Morooka impressions they did always got me. 
 
"Cause we'll be doing the panty check and I'll know when you're unclean" 
 
XD

I was always partial to Jeff's exclamations of pure surprise and shock.  Those were always good for a laugh.

" the Morooka impressions they did always got me.  "Cause we'll be doing the panty check and I'll know when you're unclean"  XD "

" @C2C said:

" the Morooka impressions they did always got me.  "Cause we'll be doing the panty check and I'll know when you're unclean"  XD "

This.  Also, "Oh, what you really mean is, will you please beat the shit out of me, Kanji?" "

"Nothing stops the Endurance Run, motherfucker!", which basicly became the unofficial tagline of the series.

What about "You made a pee-pee joke... KA-CHING, level up!" when the bunch had re-entered the first area and were talking about how Yosuke peed his pants?
 
oh, and  "Cuz I'm a CREEPY RAPER GUY!!!" is great.

This is like Dave Lettermans' top ten except with persona 4.
 
Also, you can't forget, "bitches."

One of my fav quotes is:  
Jeff: "Ryan were gonna go summon Satan!" 
Ryan: "Have fun kids!"

where the hell is "popularity leads to intimacy, Vinny"

Nanako:  "I got sick, so I had to poop in the corner."

one i can't believe i forgot...maybe it's just me that finds it as funny as i do.. but when they were talking about Skull Candy 
"DUDE, it's skull candy, cuz like, it goes in your skull! SICK!" 
i was laughing so hard at that 
also, thank you everyone else who contributed here. i love it

Getting bent is fine enough, but I loved it when they got creative with it.  "Dude, I'm bending all over the place!"
 Also, any of their many drug jokes.
Nanako:  Oh, you finally came down.
Jeff:  Yeah, I was sweating bullets for a good 8 hours there.   I was watching TV and everyone's skin melted away and there were skeletons talking to me.
Nanako:  I got a call from dad.  He said somebody gave him a watermelon.
Jeff:  Uh, maybe I didn't come down.

" the Morooka impressions they did always got me.  "Cause we'll be doing the panty check and I'll know when you're unclean"  XD "